Titanium dioxide is a well known material having desirable pigment properties and useful in paint and coating compositions and in plastics materials. Several different processes are known for manufacturing titanium dioxide material including, for example, the sulphate process and the chloride process. The present invention concerns the manufacture of titanium dioxide by the sulphate process.
The usual sulphate process for the manufacture of titanium dioxide involves the initial step of reacting a titanium bearing ore such as ilmenite with concentrated sulfuric acid (e.g., 90%-96% sulfuric acid). The reaction is sometimes referred to as "digestion" or "ore digestion." The digestion reaction of ilmenite ore and concentrated sulfuric acid is exothermic in nature and proceeds very violently. Typically, the ilmenite ore and the concentrated sulfuric acid are placed in a reaction vessel called a digestion tank. Water is usually added to the digestion tank to initiate and accelerate the acid-ore reaction because of the generation of a large quantity of heat which results in a vigorous boiling action of the water-acid solution at about 100.degree. C. to about 190.degree. C. and the release of vast quantities of steam and vapor having entrained particulate material. As the violent reaction proceeds, water is expelled and the reaction mass becomes solid; the reaction is completed in the solid phase at a temperature of approximately 180.degree. C. The solid reaction mass, referred to as a "cake," is allowed to cool. Thereafter, the solid cake is dissolved with water or dilute acid to provide a solution of sulphate salts of iron, titanium and other trace metals present in the ilmenite ore. The digestion operation is a batch procedure carried out in a single digestion tank. As many digestion tanks are used as necessary according to the desired capacity of the manufacturing plant.
After digestion, the resulting sulphate salt solution (i.e., of iron and titanium, etc.) is further processed by known measures to remove the ferrous sulphate, usually referred to as "copperas," to provide a solution of titanyl sulphate which, upon hydrolysis, yields hydrated titanium dioxide. The titanium dioxide hydrate is usually subjected to a calcination treatment in a suitable kiln device to remove the water of hydration and to provide the anhydrous titanium dioxide pigment. The foregoing process is described in greater detail in, for example, U.S. Pat. Nos. 1,504,672; 3,615,204 and 3,071,439.
The sulphate process for the manufacture of titanium dioxide pigment described hereinabove has several environmental drawbacks. For example, the violent reaction that occurs in the digestion tank results in undesirable emission problems. Also, solutions of dilute sulfuric acid, usually termed "spent acid," that result from the removal of copperas and the hydrolysis of the titanyl sulphate present severe disposal problems because large quantities of such spent acid cannot be recycled to the digestion tank, which utilizes concentrated sulfuric acid, or reclaimed on an economic basis.
Accordingly, it is the principle object of the present invention to provide a novel sulphate process for manufacturing titanium dioxide pigment that substantially avoids or eliminates the drawbacks mentioned hereinabove of the conventional sulphate process.